The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the processing of acquired MR images to alter their contrast enhancement characteristics.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mo, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The practical value of this phenomena resides in the radio signal which is emitted after the excitation signal B1 is terminated. When the excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment Mt. The frequency of this signal is the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the longitudinal magnetization of M1. The amplitude A of the emission signal (in simple systems) decays in an exponential fashion with time, t:A=Aoe−/T2 
The decay constant 1/T2 is a characteristic of the process and it provides valuable information about the substance under study. The time constant T2 is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant, and it measures the rate at which the aligned precession of the nuclei diphase after removal of the excitation signal B1.
Other factors contribute to the amplitude of the emission signal which is defined by the T2 spin-spin relaxation process. One of these is referred to as the spin-lattice relaxation process which is characterized by the time constant T1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value M0 along the axis of magnetic polarization. The T1 time constant is longer than T2, much longer in most substances. Substances can be contrasted in an MR image by the differences in either their T1 or T2 characteristics.
The measurement cycle used to acquire MR data from which an image is reconstructed is usually prescribed to enhance the contrast between tissues by exploiting the differences in T1 or T2 characteristics of those tissues. One such pulse sequence is shown in FIG. 2. This sequence, commonly known as a spin-echo sequence, performs a slice selection by applying a 90° selective RF excitation pulse 30 in the presence of a slice-select gradient pulse 31 and its associated rephrasing pulse 32. After an interval TE/2, a 180° selective RF excitation pulse 33 is applied in the presence of another gradient pulse 34 to refocus the transverse magnetization at the time TE and produce an echo NMR signal 35.
To position encode the echo NMR signal 35, a read-out gradient pulse 36 is applied during the acquisition of the NMR signal 35. The read-out gradient frequency encodes the NMR signal 35 in the well known manner. In addition, the echo NMR signal 35 is position encoded, by a phase encoding gradient pulse 37. The phase encoding gradient pulse 37 has one strength during each echo pulse sequence and associated NMR echo signal 35, and it is typically incremented in steps through a prescribed number of discrete strengths during the entire scan. As a result, each of the NMR echo signals 35 acquired during the scan is uniquely phase encoded.
There are two significant variables in such pulse sequences that affect the T1 and T2 contrast of tissues. These are the echo time TE and the transmit repeat time TR. The TE time is very sensitive to the T2 decay of the NMR signals produced by the subject tissues and its prescribed value is used to contrast between tissues based on T2 differences. The TR time is the interval between repeats of the pulse sequence and its value is very sensitive to the differences in T1 time constants of the tissues being imaged.
The trick is to set the TE and TR of the imaging pulse sequence such that maximum image contrast can be achieved between the tissues of interest. Because scanner time is very valuable, typically the clinician acquires one image with these parameters set to values which are known to produce good contrast between the tissues of interest. For many reasons, the best results are often not achieved and either the clinician makes due with the acquired image or repeats the scan with a different prescribed scan. The latter action is time-consuming and costly.